1. Field of the Invention
The present invention relates to four-wheel drive vehicles including a pair of right and left main drive wheels to which a driving force of a driving source is always transmitted, and a pair of right and left auxiliary drive wheels to which the driving force of the driving source is transmitted via a drive shaft that transmits the driving force in the longitudinal direction of the vehicle, and control devices for the four-wheel drive vehicles.
2. Description of the Related Art
Conventionally, there are four-wheel drive vehicles that can switch between a two-wheel drive mode and a four-wheel drive mode. In the two-wheel drive mode, a driving force of a driving source is transmitted only to either the front or rear wheels. In the four-wheel drive mode, the driving force of the driving source is transmitted to the front and rear wheels. This type of four-wheel drive vehicles includes a four-wheel drive vehicle in which clutches are placed on the front and rear sides of a propeller shaft that transmits a driving force in the longitudinal direction of the vehicle and both clutches are disengaged in the two-wheel drive mode to stop rotation of the propeller shaft in order to reduce driving resistance in the two-wheel drive mode and improve fuel economy (see Japanese Patent Application Publication No. 2010-100280 (JP 2010-100280 A).
The four-wheel drive vehicle described in JP 2010-100280 A has a multidisc clutch and a dog clutch. The multidisc clutch is placed on the front side of a propeller shaft (torque transmission section) that transmits a driving force in the longitudinal direction of the vehicle. The dog clutch is placed on the rear side of the propeller shaft. In the two-wheel drive mode, both the multidisc clutch and the dog clutch are deactuated to stop rotation of the propeller shaft. In the four-wheel drive mode, both the multidisc clutch and the dog clutch are actuated to transmit the driving force to auxiliary drive wheels. The multidisc clutch and the dog clutch are operated by an electric motor that is controlled by a control unit.
For example, the four-wheel drive vehicle is switched from the two-wheel drive mode to the four-wheel drive mode in the case where any of the main drive wheels to which the driving force is transmitted slips in the two-wheel drive mode or in the case where the driver performs a switch operation etc. to send a command to switch the four-wheel drive vehicle from the two-wheel drive mode to the four-wheel drive mode.
The dog clutch has a rotary member with recesses and a rotary member with projections and transmits a driving force by engagement between the projections and the recesses. These rotary members cannot be coupled together unless rotation of one of the rotary members is synchronized with rotation of the other. In order to switch the four-wheel drive vehicle from the two-wheel drive mode to the four-wheel drive mode, the multidisc clutch is first actuated to rotate the propeller shaft so that the rotary members of the dog clutch are synchronized in rotation by the rotation of the propeller shaft. The dog clutch is then actuated to couple the rotary members together.
In the multidisc clutch, however, there is a clearance between a plurality of clutch plates when the multidisc clutch is not actuated. A rotational force therefore cannot be applied to the propeller shaft until the clearance is eliminated. When the electric motor for actuating the multidisc clutch is controlled so as to quickly synchronize the rotary members of the dog clutch in rotation, the electric motor rotates at a high speed, producing noise and vibration. Such noise and vibration may bring the driver and an occupant(s) of the four-wheel drive vehicle a feeling of discomfort and insecurity. If the electric motor for actuating the multidisc clutch is rotated at a low speed, it takes a long time to synchronize the rotary members of the dog clutch in rotation even when the four-wheel drive vehicle need be quickly switched from the two-wheel drive mode to the four-wheel drive mode, such as when any of the main drive wheels slips in the two-wheel drive mode. Accordingly, rotating the electric motor at a low speed may reduce driving stability.